1. Field of the Invention
The present invention relates to a variable attenuator, a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
The radiation sources typically used with a lithographic apparatus include pulsed laser sources. Typically, for mask-based lithographic apparatus, excimer lasers are used and several tens of laser pulses are used to expose each pattern on a part of a substrate. A problem with excimer lasers is that there is a random variation of the pulse energy of plus or minus 10% of the intended energy for each pulse. However, by using a fast control algorithm and the fact that the exposure dose on the substrate is typically made up from 40 to 60 pulses, the variation of the energy dose received at the substrate is typically of the order of plus or minus 0.1% or below.
In a maskless apparatus, the pattern set by the patterning device can be imaged onto the substrate using a single pulse of the radiation system. This is because the size of the image projected onto the substrates at any one instant is relatively small and in order to provide an adequate throughput of substrate through the lithographic apparatus. However, for a single pulse, as discussed above, the energy variation can be plus or minus 10%. Such a variation in the energy of the pulse results in an unacceptably high variation in the line width produced on the substrate.
In order to provide the required radiation dose control, it has been proposed for maskless lithographic systems to employ an arrangement in which the total dose of radiation is comprised of a main pulse and one or more correction pulses. In such an arrangement, the main pulse provides the large majority of the dose of radiation. The energy within the pulse of radiation is measured and it is subsequently determined how much additional radiation is required to provide the required dose. A correction pulse is then provided in which the radiation source is set to provide a second full pulse, but the pulse is passed through a variable attenuator that is set to reduce the energy within the pulse to the required level.
For example, the main pulse can provide 90% of the dose required. Accordingly, for the correction pulse, the attenuator is set to reduce the energy in the correction pulse, such that it only transmits the energy required to complete the dose of radiation. If the radiation source generates pulses that nominally provide 90% of the total dose required, the attenuator is set for the correction pulse to only pass one ninth of the pulse of radiation, e.g., to provide the final 10% of the required dose.
Such an arrangement can allow for the energy within the main pulse to be accurately measured. The attenuator can be set to pass accurately a requisite portion of the correction pulse. However, the potential error in the dose provided by the correction pulse (which originates from the variation in the energy in the pulses generated by the radiation source) is also reduced by the attenuator. Accordingly, the overall accuracy of the dose is improved.
A further improvement can be provided, for example, by utilizing a third correction pulse in which the intensity is further attenuated. For example, the main pulse can nominally provide 90% of the dose, the first correction pulse can nominally provide 9% of the dose and the second correction pulse can nominally provide 1% of the required dose. Such an arrangement can provide a dose accuracy that is one hundred times better than the dose accuracy of the radiation source.
However, for such an arrangement to be usable within a lithographic apparatus, the variable attenuator must meet high performance criteria. Firstly, the variable attenuator must be capable of being set very accurately to given levels of transmission. Secondly, the variable attenuator must be able to switch between different transmission levels very quickly (the time between successive pulses can be of the order of 166 μs). Thirdly, the variable attenuator should be switchable between a relatively high level of transmission and a relatively low level of transmission. Fourthly, the variable attenuator must be able to operate stably for as long a product lifetime as possible. Finally, the variable attenuator must be able to meet these performance criteria for radiation at the wavelength to be used in the lithographic process. A variable attenuator that meets these performance criteria is not presently known.
Therefore, what is needed is a system and method for providing a variable attenuator that meets the performance criteria necessary for use in a subsystem of a lithographic apparatus used to control the dose of radiation in a lithographic exposure process.